Electrical network



Jan. 19 1926. 1,570,215

Y. T. c. FRY

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T. C. F RY ELECTRICAL 'NETWORK Filed June 11. 1921 asheos-Sheef 2 *f7-76 .ISL-. Usui..

Jan, 19 ,11926. I y

T. C. FRY

ELECTRICAL NETWORK Filed Jne l1. 1921 'Hy/0 |MDEDANCE=V RESISTANCE DEAcTANcE 4 CUT-orf Fnmuriwcv Dy. C C /y F Allv C A i 1,570,215 T. c. FRY l ELECTRICAL NETWORK Filed June l1. v1921 6 Sheets-Sheet 4 Jan. 19 1926.

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' 1,570,215. T. C. FRY

ELECTRICAL .NETWORK Fl'ed June 11. 1921- 6 Shasta-Sheet 6 1 /271/6/ 7 far: Ma/#aff i ffy Patented Jan'. 19, 192.6.

UNITED STATES PATENT? oFFicE.

THORNTON 0.1m?, on WYOMING, NEW aansluit, AssIGNoa 'ro WESTERN ELEc'rnxo COMPANY INCORPORATED,

application mea :une 1i,

To all whom t may concern.'

Be it known thatI, THORNTON-.Cz FRY, a citizen of the United States, residing at Vyoming, in the county of Essex, State of New Jersey, have invented certain new and useful Improvements in Electrical Networks, of which the following 1s a full, clear, concise, and exact description.

The invention relates to electrical networks built up of impedances having values such that the networks offer a desired impedance to the flow of altern-ating current.

An object of the invention is to provide a network having a specified impedance. A network of this sort is applicable to the solution of several problems arising in the field of communication. There are three general types of such problems which require in their final analysis the construction of preassigned impedances. The first type arises in designing'balancing networks employed in two-way repeater circuits, 'for example. Here the impedance to be balanced is known, and the problem is to\ duplicate this impedance as accurately as possible at all frequencies.

The second problem arises in adapting the iinpedances .of two component parts of a circuit to one another so' as'to improve thev electrical properties jof the circuit in some respect, as, for instance, by reducing reflection losses. In this case the impedances of the component parts are fixed, and cannot be altered, but are different from the inipedances which they should have in order to secure the most satisfactory'operation of the circuit as a whole. 'The problem is to design a network which when combined with the original network either in series or in parallel will give to the combination the desired impedance. For instance, the added network may be in the nature of au impedance corrective net Work which when combined with a wave filter of the Camp-- bell type provides a network which has Athe sameimpcdance at various frequencies.

Another roblem arises in correctin for the distortion which a signal experiences in transmission through a given system by OF NEW YORK, N. Y., A. CORPORATION OF NEW'YOBX.

ELEC'JERICJAI.A N ETWOBK. i

1921. Serial N0. 476,674.

addingappro riate apparatus at either the receiving or t e sending end. `In this case there is provided a network havingsuch an impedance that the output current of the distorting system when connected to this yimpedance will be of the same'wave form as the input E. M. F. of the system. R

The object of the invention 1s obtained,

and a. network of the required characteristics secured by starting with a network, the impedance of which roughly approximates thev For further details of the invention refer` ence may be had to the following detailed description and the accompanying drawings in which Fig. 1 illustrates the general arrangement of the network of this invention which is capable of possessing an impedance which ma be expressed in the form of a continue fraction as hereinafter described; Figs. 2 to 9 illustrate specic types the general network of Fig. 1 may have; Fig. 10 represents curves showing the impedance characteristics of a high pass Wave' filter; Fig.y 11 shows how an impedance corrective network according tothis invention may be addedto a wave filter to form a constant impedance arrangement Figs. 12 and 13 illustrate specific `forms of corrective networks which may be employed in Fig. 11; Fig. 14.- illustrates curves show ing the impedance characteristics and Fig. l5 the admittance characteristics of a network, the characteristics of which it is desired to make more constant with frequency; Fig. 16 shows a corrective network of the form of Fig. 4 connected in vseries with a line; Fig. 17 shows how this invention may be employed in cable duplexing; Fig. 18 represents particular values a corrective network for Fig. 17 may have; Figs. 19 and 20 show the impedance characteristic curves of the system of Fig. 17; and Figs. 21, 22 and 23 illustrate three different ways the The construction o .networks Iza/ving prenssz'gne mpedances.

The following sections give a method of systematic design of a network having a preassigned impedance, and indicate several types of networks which fulll the required conditions. v

General theory/ It follows from the general law of addition of impedances and admittances that the impedance of a network such as that shown in Fig. 1 of the drawings can be expressed as a continued fraction of the form If, then, a function F(p), where 19:211- 'i The constants a which appear in (2) are given in terms of An and` Bn by Stieltjes by the formulae If, the as are all positive, a comparison between (1) and (2) gives us Y Z1=a, c resistance,

Z3=li a capacity,

Z3=a2, a resistance,

Z4--4-1-Q c capacity,-

. asp

and so on. The network which'v possesses the impedance F(p) is then the one shown in Fig. 2.

times the frequency, f, can be expanded in a continued fraction, the elements of which represent possible impedances and admittances, a direct comparison of this fraction with (l) will furnish the values of the successive impedance elements in a network which will have the impedance 1?(27).

The expansion of a function in a continued 'fraction of the type has been studied by Stieltjes1 in a memoir in which he considers extensively the case where all of the coefficients are positive. Briefly stated, the method of expansion as given by Stieltjes is as follows:

The function F(p) is first developed in a series in terms of descending'powers of p,

which may or may not be convergent. From the coefiicients of this series are built up the quantities2 Stieltjes proves that the continued fraction (2) converges to the limit F(p), provided all of the as are positive, and the series is divergent. This theorem has for the purpose of the discussion in hand only a limited interest, however, since many-perhaps even most-of the design problems of real impoi-tance lead, in theirst instance to continued fractions having some negative elements. One of the developments of this type which recurs frequently in filter investigations has a certain number of positive as followed by two negative, two positive, two negative, and so on. This sequence of signs may be easily dealt with by means of the identity given by Stieltjes I) b v Z-ilvlE-Z'l'bi 6TH" 1Annales de la Facult des Sciencesde Toulouse. Volumes VIII and IX.

It is assumed that A=B=L To make use of this identity the fraction 2) is reduced to the following form by ividing the numerator and the denomina.-

tor of each fraction in'equation (2) by the ypartial denominator,

' r2 FUD-00+; 1

1 www1 Let the first j as be positive; and let the succeeding pairs alternate in sign. Then 1J-1a: 0, (6^) ai G14-i 0; aj+iaj+z -0a aj+zlli+a 0:

M+, denoting the remainder of the fraction. Since the term is positive, one of the troublesome negative ator B and .so on. That is, every second partial numerator, beginning with is negative. Considering the part of the fraction beginning with the partial numer- L. i li-lai? there results'from the use of (6)-, interpreting Z in lequation (6) .as

b1 a8 p and =j+g as the remainder of the continued fraction from the yterm having the partial numerator signs has been eliminated. It has, however, reappeared in the term v 0101+129 in the denominator. This, however, does not necessarily give rise to any diiiiculties, for repeated application ofthe formula leads to:

In this form of expansion every partial numerator is positive, and every artial dethe iii-st term of which is positive, the second negative. If then the numerator and denominator of each :fraction in succession by its numerator:

iu which, as has already been noted, every coeflicieut is positive. This expression corresponds to a network of the type shown in Fig. 3.

T m'i'ons types of networks obtained from the general theory.

The networks of Figures 2 and 3 are not the only ones which can be obtained from the preceding general theory. To see this, consider the expansion of F (p) 1n ascending powers of p; that is, in a series according to Taylors theorem actly the same form as (3). Hence it can be expanded in a continued. fraction (2) in terms of s as shown by Stielt]es; and this can be re-written by replacing z byzl) as Furthermore, four more types of network l may be obtained by the process of replacing impedances by admittances. Thusfif'F (p) represents the admittance of a network, and equation (3) its power series expansion, the continued fraction representing the impedance of the network will-be obtained byV equating the reciprocals of the right hand Comparing this with (1) there is obtained the network of Fig. 6, provided the as are all positive. For the other set of conditions upon the as, the network is that yshown in Fig. 7. In case the iirst negative a is a, this network degenerates into that-'of Fig. 3.

Similarly, for the expansion of the admittance in ascending powers of p, the networks of Figures 8 and 9 are obtained. Un-

`der special circumstances, the networks of Fig. 5 and Fig. 9 may be alike.

This concludes the purely theoretical part of the method of design as at present developed. We shall now take up in detail the three uses which were suggested above as typical of the fields in which the networks disclosed in this application are of value. In order that, the illustration may be as complete as possible we shall compute in each case a speciie numerical problemwhich will serve the purpose of explaining how the magnitudes of the impedance elements, from which the networks are constructed, can be X obtained.

Balancing networks.

It will be convenient in this portion of the specification to use the notation adopted and adequately explained in a paper by the inventor entitled The solution of circuit problems, which appeared in the Physical Review, Vol. 14, August 1919. In this notation the impedance of a smooth line is Z Ml e) \/p Hc KL CR where K, L, C and R are the shunt conductance, the inductance, the capacity and the resistance per unit section of a smooth transmission line if the line is so long that the amplitude at the sending end of currents relected from the receiving end is negligibly small. Let us design networks of the types shown in Figs. 2 and 8 to balance such a line. For the purpose of obtaining a network of the type shown in` Fig. 2, it is necessary to expand the impedance function in a Taylors series in descending powers of p. Such a series is readily obtained by the use of the binominal formula and is found to be where 7c equals Z =1 2 cJ-l (p) +igzxl g f i where c, 1, the coefficient of the term in g is given by the formula and left hand members of equation (2).

1 l FQ?) I a0 1 1 alp aap -l'a4 c vs E The symbol El represents the summation of the expression givenfroin values of g- 1 to g= The symbol I is the. gamma function well known to mathematicians and tabulated in many places as for instance Jahnke and Ende on Funktionentafeln init Formeln und Kurven, B. GHTeubnerLLeipil y 7 gllV'hen these vos aresubstituted in equations (4 and (5)- it is found by direct evaluation t iat i caen-T mm o The symbod Cb signifies the number of combinations of a things taken Zi at a time. l.

These as are the ones in terms of Awhich the resistance and capacity elements of the network ofy Fig. 2 are expressed. When `they are given the values assigned to them by equa-tion (12) the network of ,Fi0.`2 will, according to the theory which we have developed, have exactly the same impedance for all frequencies as has the smooth line which it is designed to simulate. Theoretically, of course, the network required to produce this resultl contains an infinite number of sections. However the networks obtained b using only a limited number. of sections ave approximately the same iinpedance as has the innite network itself.

That this is true can be inferred from the.

tions whatever is used a certain residual unbalance must remain. What actually happens is that'as the number of sections is progressively increased this residual unbalance decreases continually. In practice the minimum number of sections which will suilice to furnish any required degree -of balance may be found by determining either by computation or `by laboratory measurement the impedances of networks of various numbers of sections and noti'n under what circumstances the difference etween these im dances and that of the smooth line which is to be balanced falls within the a1- lowable limits. f

It not infrequently liappxens that the cir- 4 cuit requirements are suc that a greater amount of unbalance is allowable at certain frequencies than at others. Such requirementsmust, of course, be taken into account m determining how com lex a structure to build. They are alsol ftiiequently of importance in determinin vwhich of the various types of networks isclosed in Figs. 2 to 9 is most suitable for the purpose rat hand.Av This is illustrated to some extent by the network which we have just obtained and the network of the type shown in Fig. 8 for which the as will presently be computed. A little consideration of the structure of the networks of Fig. 2 serves to show that the convergence of the impedvences of networks of successively greater numbers of sections toward the limiting impedance of the innite network is much mere rapid at high than at'low frequencies. On the other hand the convergence in the case of networks of thety e shown in Fig. 8 is more rapid at low t an at high frequencies. Hence lwhile any requirements 'whatever as to the degreeof balance to be obtained can be satisfied by using a network of the type shown in Fig. 2 havin asuflicient 'number of sections, it is a together possible, particularly where low frequencies are of considerable importance, that a smaller number of sections of the type shown in Fig. 8 will serve the purpose' equally well. As a second example, then,` let us find the elements necessary for the construction of this kind of structure.v

For this purpose it is necessary to expand the admittance,

in a series in ascendinggiowers of p. The series thus found by the inomial expansion theorem is 1 El se-117') where the coeliicient of the term in pg is fe Per By making use of lthese constants iiiv the computation of the `determinants A.. and Bn ion and proceedinghin exactly the same' manner as in the prece 'ng illustration we arrlve at the expressions: v

for the coeiicients'of the various terms of our continued fraction.

These constants determine the sizes of the impedance elements from wh1ch the .network indicated schematically in Fig. 8 1s to be constructed. This network is theoretically infinite, as was the case with the network of Fig. 2, and when used 1n this form it also possesses an impedance which agrees absolutely with that of the smooth line at all fre uencies. Whlch type 1s to be used must e determined, as was explained above, bythe physical requlrements to be im osed upon the apparatus.

All o the computed values of the 1mpedance elements for both types of structure are expressed in the theoretical system of units to which reference was made at the beginning of this section. In order to reduce them to practical units it is only necessar to make use of the following four relat1ons,in which R, L, C and K are the resistance, inductance, capacity and leakage of the line per unit length (l) The Apractical unit of res1stance 1s times as-great as the theoretical unit: and therefore the resistance of any part of a circuit in practical units is times the resistance of the same part of the circuit in theoretical units.

(2) The practical unit of inductance 1s times as great as the theoretical unit and therefore the inductance of any part of a circuit in practical units is therefore the capacity of any part of a circuit in practical units is times the capacity of the same of the circuit in theoretical units. Y

(4) The quantity lc is KL/CR.

1t is probably wise to point out, before concluding this section of the speci-cation, that the impedance of any smooth line Whatsoevcr for which R, L, C and K do not vary with the frequency is expressed by the function Z of which we have made use in the precedinor paragraphs. The differences between the .behavior of such lines may, of course, be numerous but these dierences are taken account of entirely by the fact that the theoretical system of units in terms of which all of the computations have been made depends upon the line constants of the particular line under consideration Vand therefore itself varies from line to line. Thus the networks which We have'obtained are perfectly general and apply to all smooth lines providedonly that the proper factor be used in converting the computed resistances and capacities from the theoretical to practical units.

I mpedancc cowect'i've net/works.

In developing asystem of circuits to perform certain special functions it is not iufrequently desirable that the complete system or parts of it should possess an impedance which varies in some special man-1 ner with the frequency.

Suppose that such. a problem exists and that a network, which satisfies every requirement except that as to impedance, has

been obtained. It then becomes necessary designed is Zn and that the desired imped' ance is Z; each of these of course being allowed to vary in any physically possible manner with the frequency. By adding Van auxiliary network with an impedance Z=ZZn in series with Zx1 itself a combination is obtained the impedance of which is Hence, in order to satisfy the requirements of the problem it is only necessary to find a network having the impedance ZC. This can, of course, be done by the methods explained in the'earlier sections.

- that the current iow throuo'h them is pro- Siniilarly by bridging across the terminals'df Zn an impedance zg Z.l Z.

val shunt combination is obtained of which the impedance is 2. Hence, it is also possible by designinga network with this impedance Zo to satisfy the requirements of the problem. Y v

As an example of networks to be placed in series with the impedances which they are required to correct let us consider the design of a corrective network for a simple hieh pass filterof the Campbell ty e. Such e a 'Iter is shown in Fig. 11, with t e correct i prising a shunt inductance, L, a shunt resistance, R, and a series capacity, C, may be regarded as the impedance of-a single section terminated by the impedance Z, so that lthe impedance ofthe whole network may be written by equating Z equal to the sum of admittances -of the two paths, one'including yL and R and the ,otheaiacludingzc and z.

Sol'ving the equatioajust givenmlfor Z we ge i This varies 'widely` with .frequency as is shown by thetypical'curves o f Figure 10. Such a varlation of impedance is frequently undesirable and a-network which will reduce it to a constant resistanceis vhighly desirable. 1" v Suppose we desire to make, .the resistance of the combination equal to the characteristie impedance. l

of the lfilter. That is,- we wish to have i as shown in the acuta une einig. 10. n then becomes necessary to find a network of whichthe impedance is ZoslZ-Zne By expanding this function in descending powers of p by means of the binomial theorem and from the coefficients thus obtained from equations (4) and (5), we obtain the following continued fraction To this corresponds the. network shown in Figure 12.

This network when connected in series with the lteras shown in Fig. 11 accurately corrects the impedance of the filter for all frequencies .whatever the values of R, and C. If, however, L is less than RZC some of the values assi ed to the resistances are negative, that 1s, resistances, such portional to the electromotlve. force across them, but in a dlrection which transfers positive electricity from the point of low potential to that of high potential. Since .such negative resistances are not at present available it is not physically possible to construct the network when this relation exists between the constants L, R and C. A net'- 'work which can be constructed, however, when L is less than RZC may be obtained by the expansion of Zc in ascending powers of p. This expansion is carried out by means of the binomial theorem and when its coeicents are substituted in equations (4) and (5) leads to the continued fraction Zo Z 'i' 1 y where 2R01 1 five-L a" a? I aalia etc.

-. most any type of impedance correction, that is, for given impedances Zn and desired im-V m pedances Z which vary in almost any man- 3 ner whatsoever with frequency. 0f course,

it may happen as in the first example above for L Iless than RZC that. negative' resistances, inductances or capacities are involved in the design. When this occurs it indicates that the conditions are'such that they cannot be satisfied by any network of the type under consideration which is composed only of such elements as are available. Un-

der these circumstances 'it is necessary to investigate the possibility of the .use of other types of networks as was done 1n the example above. Since the actual computatlon of the impedance elements out of which the network is to be constructed involves a considerable amount of algebraic and arithmetical work, it is highly desirable to have a method of eliminating in advance those types of network which cannot satisfy the conditions 3e of the problem without the use of negative impedance elements. How this may be done can best be illustrated by the considerationV of a Special case.

Suppose the impedance which is to be cor-` 35 rected is that shown in Figure 14 and thatsome means is sought of altering the system giving riseto this impedance in sucha manner as to give the altered system a pure resistance. We wish to determine which of the network types of Figures 2 to 9 can be used for this purpose.

Attention is called to the fact that the reactance component of the impedance to be corrected is very large and negative at low frequencies. If, therefore, the corrective network were to be placed in series with the impedance to be corrected it would be necessary that the impedance of the corrective network have an equally large but positive reactance component. But it is Well known that physical systems composed only of positive resistances, inductances and capacities cannot give rise to a reactance component which varies in this manner with the frequency. Hence we may conclude that no corrective network can be obtained which, when attached in series with the impedance shown in Figure 14, yields a combination which has an impedance equal to a pure resistance.

If the corrective network is to be attached lin parallel with thefilter network then it is the admittance which is of importance. Let us, therefore, consider the admittance curves,

which are shown'in Figure 15, and investi- Similar networks can be designed-for al-l .gate whether orl not any of the typesfof netis representedb the dotted line markedI A1. In order that t is admittance may be realized it is necessary in the first placethat the susceptance of the corrective network shall be equal to the susceptanee shown in Figure 15 but opposite to it n sign, and in the second place that the conductance of a corrective network shall be of just such a magnitude at each frequency as to build the conductance curve of Figure 15 u to the line A1. Since in the neighborhoo of the frequency 2,000 the conductance curve lies above A1, this would require that the con' ductance of the corrective network be negative in the nei hborhood of this frequency. Now it is well nown that no'network composed entirely of positive resistances, inductances and capacities ever gives rise to a negative conductance. We must, therefore, infer that there is no type of network which can satisfy thefconditions laid down.

If, however, the admittance is that represented byA2 this objection does not apply and a network involving positive elements only may be obtained. v

Let us now consider one or two of the network types shown in Figures 2te 9 and see whether they possess in the neighborhood of zero frequency the proper type of admittance to correct the curves of Figure 15 in the desired manner. Consider first the network of Figure 2.- Alittle investigation serves to show us that bot-h the c nductance and susoeptance of this net-work appoach zero as zero frequency is approached.`

oreover, the si of the susceptance is positive. Neither o these values is satisfactory since the network which is needed to correct the impedance curve of Figure 15 must have a finite conductance and a small but negative susceptance. In the case of the network shown in Figure 3 the conductance component is finite at zero frequencies but the susceptance. is still positive.v fThis net-` this type can be so designed as to satisfy all the requirements which we have laid down.

The impedance curves of Figures 14 and 15 are of no consequence except in so far as they serve to illustrate the sort of argument by means of which certain types of networks may be eliminated from consideration and the actual computation restricted to those which give promise of requiring onl positive elements. There is, therefore, no urther need of considering these curves and we shall pass on to a consideration of certain novel arrangements of balancing networks.

Impedance corrective networks for smooth Zmes. l l'n an earlier division of this specification the construction of balancing networks for smooth lines was considered in some detail.

The present section will be devoted to a consideration of some impedance corrective networks for such lines.

As a. first example let us consider the expansion of Z(p) from the first equationv appearing under the above section entitled Balancing networks in a series in ascendl ing sewers of p. This expansion is carried out y the binomial theorem and leads to Zewesp-J,

where f I Perec This leads to a continued fraction of the form in which the coefiicients are found from the direct evaluations of equations (4) and (5) to be given by the general formulae -a2n= l n n-'l Y ai 2 egyptroca-1km 14) 24% i Cifkm 2 Let us now investigate the possibilit of adding to this line in series a network w ich will correct its impedance to the value s The impedance of the corrective network i This corresponds to a network of the type l shown-in Figure 4 in which aio is to be given fthe value zero and the remainder of as the values written above.

- When such a network is combined in series with the transmission line as shown in Figure 16 the resulting combination has at all frequencies the impedance In practice 7c is always very small so that all l the coeicients in the continued fraction and therefore all the impedance elements of Figure 16 are positive and the network can actually be constructed of impedance elements in which 1 ZD-l If we attempt to find a network which,

when placed in parallel with the line, will Application of z' yield a combination having the impedance 'Z'=1 we find that the impedance of this corrective network must be v,represented by thejcontinued fraction anp+ 0 This network is of-'the type shown in Figure (5, a., being zero and the remainder of the as having the values assigned to them above. All of these constants are ositive when k issuiiciently small, from whlch fact it follows that thisnctwork also can be constructed of impedance elements which are physically obtainable. y s It is scarcely necessary to remark that all these computations lhave been carried out in the s )ecial system of units introduced in the sectlon on Balancing networks and can be reduced to ordinary units by the rules given in that place.

We have thus found in the resent section two networks either one 'of w ich will serve the purpose, when placed in combination with a smooth line, of rendering the impedance of the combination equal to a pure resistance of the same value at all frequencies.

4edf/tace corrective networks to ba ancing problems.

The idea involved in most balancin systems is essentially the same; name y, to build up a bridge arrangement having for one of its arms the transmission line or cable, for another arm a network which simulates as closely as possible the impedance of the line or cable, and for the remainmg arm two impedance elements similar to one another but different from the other pair. Si nals arriving over the line are detected by means of a receiving apparatus connected between the points at which dissimilar arms meet, while outgoingsignals are 'impressed upon the system by means of voltages applied between the points at which similar arms meet.

It is equally possiblehowever, to use as one armof the bridge the combination of the line or cable and an impedance corrective network of either the shunt or series type so designed as to give to this arm an impedance equal to a pureresistance at all frequencies. The remainder of the bridge will then consist of a yresistance element equalling the resistance of the arm in which the line occurs and two other arms similar to one another but not to the iirst tioned.

lhus in the case of cable duplexing, which may be taken as typicah'the arrangement would be that shown in Figure 17 if pair men- -that the last shunt resistance of the impedance corrective element of the shunt type is used.

In this figure the arms marked Z are the finite bridge arms, which in the present commercial s stems of cable signaling are condensers; Sy is the receiving apparatus; Z is the impedance corrective network and the is the fourth-arm of the bridge.

In order to gain an impression of the degree of balance which may be obtained from such a system let us consider as a numerical case a cable having the constants R=5 ohms, '020.3 microfarads, L29 millihenrys and K=0 a network of four sections will give a sufiicicntly accurate correction the values of the four sections should be in the assumed case as shown in Fig. 18. With exception of the resistance marked 572 ohms, the elements have been assigned values according to the above equations holdin for the case where a very large number o? sections were to be employed. According to these formulae the fourth resistance from the left would have a value of 2570 'ohms if other sections. of the network were to be added. In such a case as given above however where only a few sections, four for example, are to be employed, the last shunt element instead of corresponding to the formula, may be so constructed as to have an impedance Aequal to the impedance of the remainder of such a network of infinite number of sections for sqme one frequency. If for the purpose for which the network is desired, an especially accurate balance is required at some one frequenc or within some band of frequencies, as or example, within the voice range inthe case of the particular embodiments shown in Figs. 21, 22, 23, this one frequency.

. resi stance -or a frequency within this band of frequencies will preferably be chosen as that at which the last shunt element shall match.

the impedance of the remainder of the infinite network. This observation re arding the termination of he network of l'i 18 applies equally well to the other types o networks illustrated in the attached drawings.

Since the network of Fig. 18 has been illusi trated for a d.c. telegraph system it may be easily calculated from the above e uations i 18 should have a value of 572 ohms. g

When the vfou-r meshes of this network shown in the ligure are connected in parallel with the cable the impedance of the combination is that vshown in arbitrary units in Fig. 19. It will be seen that this impedance 1s very nearly a pure resistance. In order to auge more clearly the actual magnitude o the departures from thedesirecl of frequencies.

impedance, thesedepartures are plotted in- Fig` 20. 1n the same figure 'is shown the def' partures of the common type of artificial line from the impedance of the smooth line which it is designed to simulate.

In considering these curves, attention should be directed to the fact that the artificial line is taken to be infinitely long,yand is so divided that each section represents 4.08 miles of cable, so that hundreds of sections would be necessaryto represent a trans'- Atlantic cable; whereas the impedance corrective network'has `only four meshes as shown in Fig. 6. In spite of this fact, the departures for the latter are larger than those of the former at only a small range For telephonie frequencies the four-mesh network is already much superior to the artificial line. F urthermor'e, the range of inferiority could lbe cut down to any desired extent by adding sections to the network of Fig. 6 in accordance with the above theory.

This method of duplexing presents sev-- eral advanta es over the method in common use of whici the following may be mentioned:

1) In the example considered the theoretical balance obtained from the proposed system, when only four sections of network are used, is better than the theoretical balance from an artificial line of an infinite number of four mile sections, except at frequencies below 100 cycles.

(2) This limit (100 cycles) can'he reduced veryfmaterially by adding additional s ections. From a purely theoretical standpoint. it can be reduced to any frequency desired..

(3)` A very desirable ty e of frequency selectivity is introduced at oth the rece1ving and sending ends. That is, the balancing network is, to a considerable extent, a distortion corrective network as well.-

(4) The system. makes the use of condensers in the finite bridge arms purely optional, in the case of cable transmission. The cable is automatically electrostatic potentials resu ting from accumulated charges.

Telephone repeater balance can be obtained in one of three ways: (a), by the scheme shown in Fig. 21 in which figure A, A are networks simulating the impedances of the line L, L res ectively; or, (b), by the scheme shown in ig. 22 1n which B, B

are impedance correct-ive networks, which,

when combined in parallel with the lines L, L', reduce their impedances to R, R', res ectively; or (c), la' the scheme shown in ig. 23, in which C are impedance corrective networks, whlch, when Icombined in series with the/lines L, L", reduce their impedances to R, R', respectively.

After what has alread f been said, it is at once obvious that we ave designed netrotected from works for either of these balancing schemes.

The first scheme, which is the one generally in use, vrequires networks of the types developed and discussed under the heading Balancing networks, in connection with equations (12) and (13). A s was said in connection with the discussion of those equations, the networks to which they lead may be'made to simulate the lines as accurately as we ma desire by the use of a suitably large num er of sections.

he second scheme utilizes the impedance corrective network defined by e nations (15); and the 'third scheme that de ned by equations (14). To each of these schemes, neither of which has heretofore been recoginized as operative, the same remark applies,-that is, we may make the balance as nearly absolute as we may desire by choosing the number of sections sufficiently large.

In certain of the appended claims the networks of this invention are specifically referred toas two .terminal networks, thereby distinguishing from such networks of the form of a wave filter, for example, which has fo'ur terminals, two for incoming poten-v tials and two for outgoing potentials.

In further explanation of certain claims which follow, attent-ion is called to the fact that even in those. cases in the main body of the specification where it has been more convenient to s ak of expanding the admittance in a continued fraction, the result obtained ywas an expansion of the impedance in such a fraction, as may easily be se'en by equating to the impedance unity divided by that continued fraction which represented the admittance. Furthermore, in every such expansion the successive partialdenominatorsi are bi-linear functions of the quantity p, (that is, functions of the form Where a, 'y and are numerical constants), the partial denominators correspending alternately to the impedances to be assigned to the series portion of a network section, and to the admittance to be assigned to the shunt portion of a network section. The sole aim of the expansion is to produce successive partial denominators of this form, .by which the impedances themselves are uniquely determined.

What is claimed is:

1. A network having a desired impedance characteristic comprising a plurality of sections, each having an impedance which varies from section to section in accordance with the terms of a continued fraction so that each section when taken in connection with the preceding sections more closely ap- 1 proximates the desired impedance.

have values differing from section to section and corresponding to the coefficients an, al, a2, etc., of the expansion of the impedance to be simulated into a continued fraction of the form a pits 04+ wherein p equals 21m] 1 times the frequency.

3. A corrective network having only two terminals for connecting to a transmission circuit, and a plurality of sections, each having impedance elements effectively in shunt to said terminals and impedance elements in series with said terminals and the other sections, the impedance elements of cach section being similar to the impedance elements of the other sections but having values differing from sect-ion to section.

4. A corrective network having only two terminals for connecting to a transmission circuit, and a plurality of sections, each having like impedance elements effectively in shunt to said terminals andimpedance elements in series with said terminals and the other sections,vsaid impedance elements being similarly arranged in said sections but having values differing from section to section.

5. A corrective network of a plurality of sections, said sections having like impedances similarly arranged, but of values differing from section to section, each of said vsections comprising a resistance, and a react- Iance in shunt.

6. A corrective network of a plurality of recurring sections comprising impedances substantially differing in value from section to section, each of said sections comprising a condenser shunted by a resistance.

7. ln combination, a transmission line and.

an electrical network comprising a plurality of sections, said sections having like impedances similarly. arranged but differing substantially in theirrespective values from section tosection, said network at one end being connected to said line and at the other end being closed upon itself.

8. In combination, a transmission line and an electrical network comprising a plurality of sections, said sections having like impedances similarly arranged but differing substantially in their respective values from section to section, said network at its end having the impedances of the smallest values bein connected to said line and at the other end eing closed upon itself.

9. A network comprisin a connecting line, a plurality of similar impedances oonnected in series in said line, a pluralit of similar impedances in shunt to said line, thereby forming a network of a plurality of similar sections, said series impedances having values differing from section to section, and connections at only one end of said network to a transmission line said netwck being closed upon itself at the other en 10. A network of a. lurality of sections similar in im edance c aracter but having substantiall iferent im edance values in each of a p urality of sections, said network being closed upon itself atene end and having connecting terminals at the other end.

11. In com ination, a transmission line and a network of such Lan impedance thatv the combined impedance of said line and said network approximates a pure resistance, said network comprising a plurality of sections having similar impedance arrangements.

12. In combination, a transmission line and a network of such an impedance that the combined impedance of said line and said network approximates a pure resistance, said. network comprising a plurality of sections having similar impedance arrangements, the impedances in said sections having values differing from sectionto section.

13. In combination two line sections, a

two way repeater circuit between said sec-` tions,one of said line sections having an impedance with a resistance and a reactance component,--an artificial balancing line for said last mentioned section comprising a pure resistance only,-and means between ,said last mentioned section and said artificial line whereby the combined impedance of said means and said last mentioned line section approximates a pure resistance.

14. In-combination two line sections, a two Way repeater circuit between said sections,-one of said line sections having an impedance with a resistance and a reactance component,-an artificial balancing line for said last mentioned section comprising a pure resistance only,and means comprising a network of a plurality of sections having like impedances but with values differing from section to section, between said last mentioned section and said artificial line whereby the combined impedance of said means and said last mentioned line section approximates a pure resistance.

15. A circuit for changing the impedance Z,1 of an electrical device to Z, said circuit comprising a section the impedance of whic is a first approximation of 1* ZnZ Zn-Z

. panding said function, said expansion .v ing` carried out to the desired approximation.-

17. A network, the 'impedance ofwhichl has a value approximating to a function .of,

ihas a value approximating to a4 function of' two impedances, said network havin .a'plurality of sections, the impedance e 'ements to thev of which have values correspondin terms of a continued fraction ,fonn b. ex-

two impedances, said network havin' v...a pl'urality of sections, the impedancel e ements' of which have values correspond' l 7the,

, lexpanding said function, said expansion ing carried out to the desired ap roximaticn, e; impedance or said net` work-havin a va ue a proximating tlieiin-fjw44 edance of terms offa continued. fraction form and a terminati e remain er of-,an 'infinite n um' r of. sections corresponding to the.: term of said fractionif expanded -to infinity. 18. A corrective network` having tw minals and comprisin a lurality fA 'l .f tions, anch section haiing ike impedanc'es lof pwhe'mm diii'erin -in value from sectionto secton, c ach o said sections comprisin i a resist ance effectively 'in shunt to'sai two ,terminals-W- 19. In combination, two line sections, a two-way repeater circuit between said line sections, one of saidr line sections having an impedance with a resistance and a reactance component, an"artiicial balancing line for said last mentioned section comprising a pire resistance only and impedance means.` tween said last mentioned section and said artificial line of such impedance that-the combined impedance of said means andvsaid last mentioned line section approximates a d pure resistance substantially equal to the4 v ure' resistance of said artificial balancing me.

i' 2 0.' In lcombination twov line sections, a

,two..l"way',re `tions, one o sald line sections having an im.`

ter circuit between said secpedance wit f a resistance and a reactance component,.an artificial balancing line for said last mentined section comprising a pure resist-ance only, and means said section and saidartificial line and in shunt to said section whereby the combined impedance of said means and said section approximates 'aj pnre resistance.

4g'ietvvork Aati.' the end t dterminals.

22, A" network-comprising a plurality of ementsin successive sections, impedance e h vdetermined by the y, the successive pairs of 'artial' denofm nators-fof the expansion -of belsimulated mtoa conwhich are` b111near functions i2-1 times the frequency. 23:11u` com inatlonv-, a network having an impedance with a resistance component and A.a reactancel component, means having a subatantiall pure-resistance only, and a second electrica n'etlwo'rkl comprising a' plurality' of sections in circuit with the fins-t network and said means andfhaving such 'an impedance -that the combined impedance of said networks approximatesa pure resistance substantially equal in value to the resistance of said means; I`

In witness. whereof, I hereunto subscribe my name this-9th day of June A. D., 1921.

THORNTON c. FRY.

between.

421. fAv :two terminal corrective network -of ,"plnra'lity of sections having like imped imcjes' but' with.values `differing from section sectionand `aiterminati impedanceefor ereof "opposite 

